Streamflow responses to afforestation with Eucalyptus grandis and Pinus patula and to felling in the Mokobulaan experimental catchments, South Africa

Abstract

The reductions in streamflow following the afforestation of grassland with Eucalyptus grandis and Pinus patula in the Mokobulaan research catchments on the Mpumalanga escarpment, and the subsequent response in streamflow to clearfelling of the eucalypts are presented. Afforestation with eucalypts of an entire catchment with a virgin annual runoff of 236 mm caused a statistically significant decrease in streamflow in the third year after planting and the stream dried up completely in the ninth year after planting. The eucalypts were clearfelled when 16 years old but full perennial streamflow did not return until five years later. Afforestation with pines of an entire catchment with a virgin annual runoff of 217 mm, produced a significant decrease in streamflow in the fourth year after planting and caused the stream to dry up completely in the twelfth year after planting. The drying up of the streams was not altogether surprising as the annual runoff was lower than the expected reductions owing to complete afforestation. The delayed return of streamflow in the clearfelled catchment is surprising though, and is attributed to the desiccation of deep, soil-water stores by the eucalypts. These stores had to be replenished before the streams could return to normal behaviour.